refactor: update GITHUB token check in prep:release

Co-authored-by: Asher <ash@coder.com>

Author: jsjoeio

ci/build/release-prep.sh

@@ -25,7 +25,7 @@ main() {
   cd "$(dirname "$0")/../.."
 
   # Check that $GITHUB_TOKEN is set
-  if [[ -z "${GITHUB_TOKEN}" ]]; then
+  if [[ -z ${GITHUB_TOKEN-} ]]; then
     echo "We couldn't find an environment variable under GITHUB_TOKEN."
     echo "This is needed for our scripts that use hub."
     echo -e "See docs regarding GITHUB_TOKEN here under 'GitHub OAuth authentication': https://hub.github.com/hub.1.html"